1. Field of the invention
The present invention relates to an inversion prevention device to prevent inversion of FM signals.
2. Description of Related Art
When the point where an FM signal crosses the zero-level disappears, there happens the phenomenon that the demodulation signal of the FM signal being the white level in itself drops to the black level. This phenomenon is called as "inversion" hereinbelow.
As a generally known inversion prevention device of the prior art, there is a device called a double limiter. For example, FIG. 1 shows a block circuit diagram of a double limiter which is a conventional inversion prevention device disclosed in the Japanese Patent Application Laid-Open No. 62-219372 (1987). In FIG. 1, numeral 601 denotes an input terminal for reproduced FM signal. The FM signals are inputted to a high-pass filter (hereinafter called LPF) 602 and to a low-pass filter (hereinafter called LPF) 605 via the input terminal 601. The HPF 602 passes carrier wave component of the input FM signals and significantly attenuates lower sideband componet, then outputs the resultant signals to a limiter 603. The limiter 603 eliminates a difference in the amplitudes of the input signals arid sends sinusoidal output signals having a constant amplitude to a phase compensator 604. The phase compensator 604 compensates the phase of the input signals and outputs the compensated signals to a synthesizer 606. On the other hand, the LPF 605 attenuates the carrier wave component of the input FM signals sufficiently and outputs the resultant signals to the synthesizer 606. The synthesizer 606 synthesizes both input signals and outputs the synthesized signals to a limiter 607. The limiter 607 regulates the amplitude to a constant level to prevent the demodulated output from the FM demodulator later at a later stage from being distorted by AM component, then outputs the FM signals via an outpost terminal 608.
In the conventional inversion prevention device in such a constitution as described above, carrier wave component is amplified until a specified amplitude is obtained by the limiter 603 (at a cost of decreased S/N ratio) in a portion of the input FM signals where the carrier wave component is so small that an inversion is likely to occur, thus FM signals wherein only the carrier wave component is amplified and which are not susceptible to inversion are obtained at the output terminal of the synthesizer 606. In a portion of the input FM signals where the carrier wave component is large enough to prevent an inversion, the carrier wave component attains the specified amplitude without being subject to significant amplification by the limiter 603 (therefore the S/N ratio does not decrease), and consequently FM signals substantially equal to the input FM signals are obtained at the synthesizer 606. Therefore FM signals free from inversion are always outputted.
An example of a digital signal processing circuit which achieves the same functions of the inversion prevention device described above to deal with digital FM signals which are quantized is disclosed in the Japanese Patent Application Laid-Open No. 62-219372 (1987), with its block circuit, diagram being shown in FIG. 7.
This device handles digital FM signals which are quantized FM signals as an input and output. Numeral 701 in FIG. 7 denotes an input terminal for the digital FM signals. The digital FM signals are inputted to a HPF 702 and a LPF 706 via an input terminal 701. The HPF 702 outputs carrier wave component to an amplitude detector 704 and a divider 705 provided in a limiter 703 which regulates the amplitude of the output from the HPF 702 to a constant level. The amplitude detector 704 outputs the amplitude signal which represents the amplitude of the output signal of the HPF 702 to the divider 705. The divider 705 divides the output signal from the HPF 702 by the amplitude signal from the amplitude detector 704, and outputs the carrier wave component of a constant amplitude to a synthesizer 707. The synthesizer 707 synthesizes the output of the limiter 703 (divider 705) and the output, of the LPF 706, while the synthesized digital FM signals are outputted from the outputted terminal 708.
Because it is easy to make the HPF 702 and the LPF 706 operate with linear phase characteristics, a phase corrector is not necessary and the limiter 607 in FIG. 6 has no direct implication to the prevention of inversion and is therefore omitted. The basic operation in the digital inversion prevention device shown in FIG. 7 is similar to that shown in FIG. 6 and therefore the description thereof will be omitted.
However, the conventional device shown in FIG. 7 requires the amplitude detector 704. To construct the amplitude detector 704 requires a quadrature phase shifter which shifts the phase of input X to the amplitude detector 704 by 90.degree. and an arithmetic logic unit which calculates the square root of (X.sup.2 +Y.sup.2) by using a signal Y having a phase which is shifted by 90.degree., leading to a considerably large circuit.
The divider 705 having a circuit substantially larger than the arithmetic logic unit is also required. Moreover, because the output signal of the HPF 702 is a carrier wave component having a large amplitude in contrast to the output signal of the LPF 706 which is the sideband component having a small amplitude, it is necessary to increase the data word length thereof. As a result, circuits of the divider 705 and the amplitude detector 704 become even larger, causing a problem that it is difficult to make a digital inversion prevention device. In addition, while the conventional device has a function of minimizing the deterioration of S/N ratio of the input FM signals, it cannot provide a S/N ratio higher than that of the input FM signals.